In the past a power drive unit has used two hydraulic motors with output shafts connected through a gearing arrangement so that a single output shaft can be rotated by either the combined output of both motors or the output of either motor alone. Where a failure of one motor occurs and its output shaft cannot be disengaged from the drive gearing, the failed motor constitutes an additional drag on the system.
Systems of this general type find use in aircraft flight control actuators. Where the controlled element is an airfoil surface, it is desirable in some applications to provide damping of the surface movement when both motors have failed.